The present inventors have developed a method for producing an emulsion using the crossed form of microchannels as a means for producing microdroplets (emulsion) having an excellent size uniformity (monodispersity) (WO 02/068104).
This technology has made it possible to produce an emulsion of a uniform size, and to flexibly control the diameter and the formation speed of emulsion droplets by manipulating the flow rate in the channel. The technology is being used in the production of a multi-phase emulsion (Kokai (Japanese Unexamined Patent Publication) No. 2004-237177), the preparation of globular solid microparticles (Kokai (Japanese Unexamined Patent Publication) No. 2004-059802 and Kokai (Japanese Unexamined Patent Publication) No. 2004-067953), the preparation of colored solid microparticles (Kokai (Japanese Unexamined Patent Publication) No. 2004-197083) and the like.
However, the above technology has problems that, with the crossed structure of only one microchannel, the upper limit of the flow rate for producing droplets is suppressed and the volume to be treated becomes smaller. In order to resolve the problems, a number of examples on the development of apparatus in which a multitude of microchannels are arranged in parallel have been reported. For example, there is a report on a microchannel board in which a total of 3 layers are stuck comprising (a) a layer of microchannels for distributing a dispersion phase, (b) a layer of microchannels for distributing a continuous phase liquid, and (c) a layer of Y-shaped microchannels for producing droplets (Kokai (Japanese Unexamined Patent Publication) No. 2004-243308).
On the other hand, the present inventors have developed an apparatus comprising: a microchannel board in which a multitude of microchannels in a crossed form for producing droplets are arranged; and a holder for retaining the microchannel board having a hierarchical structure to control the allocation of liquid to each microchannel (WO 2007/026564, Lab Chip, 2008, 8, 287-293).
However, in a microdroplet production apparatus as described above, the holder for retaining the microchannel board must be equipped with a plurality of liquid feeding paths corresponding to the plurality of inlet ports (liquid feeding ports) for feeding the dispersion phase and the continuous phase from outside the board to each of the channels of the microchannel board. This structure has problems as described below.
First, as the number of channels to be arranged in parallel in the microchannel board becomes increased, the number of liquid feeding ports of the microchannel board must be increased. Therefore, a multitude of liquid feeding paths must be provided at the corresponding positions in each hierarchical layer of the holder for retaining the microchannel board. While it is desired, from the viewpoint of effectively using the board area, that the channels be arranged more densely on the microchannel board to increase the number of channels per unit area, this requires to minimize the size of the liquid feeding ports made on the microchannel board and the hole size of the corresponding liquid feeding paths of the holder for retaining the microchannel board. Generally, a holder for retaining a microchannel board may be fabricated by machining, but the processing of a multitude of fine holes is technically difficult and leads to high cost. Also, when fine holes are densely made on the holder for retaining a microchannel board, it becomes difficult to process the feeding ports for the dispersion phase or the continuous phase from the side of each hierarchical layer by passing through the gaps of vertical holes.
Also, a holder for retaining a microchannel board cannot be used for a microchannel board having the liquid feeding ports at different positions, and thus it disadvantageously lacks versatility.